Blow molding is a common process for forming plastic bottles and similar containers. The blow molding process typically includes heating and pressurizing the interior of a plastic preform inside a mold to expand the preform to the shape of the mold. Preforms are often formed of a thermoplastic, for example HDPE or PET, by injection molding, extrusion, or a combination of both methods.
Blow molding process include injection blow molding and extrusion blow molding, each of which generally correspond to the method of forming the preform. The injection blow molding process typically conveys, mixes, melts, and injects resin into an injection mold, often using a screw conveyor. The extrusion blow molding process may be continuous, in which molds move around the preform and then move away from the extruder, or intermittent in which the extruder intermittently pushes the extrusion in a mold similar to injection blow molding.
A typical preform 92, of the type that may be blown into a plastic bottle, is shown in FIG. 9 (prior art). Preform 92 includes an open end 94a, a substantially cylindrical body 95, and a rounded tip 97 at a closed end 94b, which is opposite the open end. The open end 94a may have screw threads 93 formed near its neck, and a neck support ring 93, which forms the largest diameter of the preform. Support ring 93 forms a shoulder 91 that includes a flat surface that faces closed end 94b. At the end of the rounded tip 97 is the gate 99, which is typically an oblate ball shape.
Preform design and production quality is crucial to forming a blow-molded container having sound physical properties. Further, because each final container design requires its own unique preform shape, quality control measures often must be modified to accommodate each preform. A critical area of quality in preform production is the integrity of the gate area. Defects in the gate area may create a defective container or cause related structural or process problems.
Defects that may occur at the preform gate include crystal penetration, excavation, and voids. Crystal penetration occurs when the plastic (most often PET) that forms the gate area is improperly heated or cooled during forming of the preform such that the plastic has a partial crystalline form, rather than the desired amorphous form. Crystal penetration causes a lack of elasticity, which may cause the container to shatter during the blow molding process. Excavation occurs often by overheating during forming of the preform, and manifests as sinks or pits in the surface of the gate. The sinks may cause the container to burst during blow molding. Voids are similar to excavations, except that voids are not on the surface of the preform, but rather are encapsulated by the plastic of the gate. Voids may also cause the container to burst during blow molding.
Without adequate inspection, gate defects may result in large amounts of improperly formed preforms and containers. Prompt gate inspection and detection of defects are especially important in mass production circumstances so that process parameters may be promptly adjusted to correct the defects. Unfortunately, detection of gate defects is difficult, especially from an external inspection of the gate.